Pavement marker for day and night visibility



July 16, 1968 i s. A- HEE NAN ETAL 3,392,539 PAVEMENT MARKER FOR DAY ANDNIGHT VISIBILITY Filed Oct. 12, 1966 2 Sheets-Sheet l INVENTORS SIDNEYA. HEENAN B GLENN W. JOHNSON, JR.

ATTOR EYS J ly 16. 1968 s. A. HEENAN ETAL 3,

PAVEMENT MARKER FOR DAY AND NIGHT VISIBILITY Filed on. 12, 1966 zSheets-Sheet z INVENTORS SIDNEY A. HEENAN BY GLENN W. JOHNSON JR.

ATTOR NEYS United States Patent 3,392,639 PAVEMENT MARKER FOR DAY ANDNIGHT VISIBILITY Sidney A. Heenan, Park Ridge, 11]., and Glenn W.Johnson, Jr., Summit, N.J., assignors to Elastic Stop Nut Corporation ofAmerica, Union, NJ., a corporation of New Jersey Filed Oct. 12, 1966,Ser. No. 586,192 13 Claims. (CI. 94-15) The present invention relatesgenerally to pavement markers and pertains more specifically to roadwaytrafiic markers which are cleaned by the action of vehicular traffic onthe roadway contacting the marker to maintain optimum visibility duringservice and which provide adequate visible markings during both the dayand the night.

Pavement markers employing reflectors have enjoyed a high degree ofacceptance in providing roadway markings visible from oncoming vehiclesunder nighttime conditions as a result of their ability to reflect lightemanating from such vehicles. While it has been observed that theoptical effectiveness of such reflectors will diminish rapidly duringservice as a result of the accumulation of dirt and other deposits whichcan form a film over the reflector system and inhibit the passage oflight, it is now recognized that various means can be provided to cleanthe reflector surfaces in response to the contact of vehicular trafiicwith the marker structure and thus maintain their optical effectiveness.However, while effective reflector systems will provide adequatenighttime visibility in such markers, even clean reflectors of therelatively small sizes adequate for nighttime marking do not ordinarilyprovide a visible marking necessary for daytime use.

It is therefore an object of the invention to provide a pavement markerfor establishing a marking which is adequately visible from an oncomingvehicle during both the day and the night and which will maintain itsvisible effectiveness during service.

Another object of the invention is to provide a pavement marker of thetype described above and which can be installed readily and economicallyupon a roadway surface and which will withstand the constant contactwith vehicular traflic inherent in such an installation.

Still another object of the invention is to provide a pavement markeremploying a reflector system for nighttime visibility and an opaquefacial area for daytime visibility, both of which will be wiped clean ofaccumulated dirt by contact with oncoming vehicular traffic and both ofwhich will withstand mechanical abrasion arising out of such contact.

A further object of the invention is to provide a pavement marker of thetype described which is self-cleaning with no moving component parts.

A still further object of the invention is to provide a pavement markerof the type described which is simple in construction, which isfabricated of relatively inexpensive materials and is capable ofeconomical installation and maintenance.

The invention will be more fully understood and additional objects andadvantages thereof will become apparent in the following detaileddescription of an embodiment of the invention illustrated in theaccompanying drawing, in which:

FIGURE 1 is a partially sectional perspective view of a pavement markerconstructed in accordance with the invention;

FIGURE 2 is a front elevational view of the pavement marker of FIGURE 1;

FIGURE 3 is an enlarged cross-sectional view taken along line 3-3 ofFIGURE 2;

FIGURE 4 is an enlarged fragmentary view of a portion of FIGURE 3; and

3,392,639 Patented July 16, 1968 FIGURE 5 is a perspective view of analternative configuration of a pavement marker incorporating theinvention.

Referring now to the drawing, structed in accordance with theperspective at 10 in FIGURE 1 a composite construction including a shell12 of synthetic resin partially sectioned to show that it is filled, orpotted, with a relatively rigid filler material seen in the form of acast solid core 14 which is contiguous with the inner surface 16 of theshell 12 and serves to reinforce the shell 12 and provide a solid,rugged structure capable of withstanding forces applied to the pavementmarker when the outer surface 18 of the marker is struck by vehiculartrafiic during service.

The pavement marker is usually employed to provide a marking on agenerally horizontal roadway surface, the marking being visible from anoncoming vehicle on the roadway to delineate traffic lanes, and, as bestseen in FIGURES 2 and 3, is thus provided with a generally horizontalbase 20 for cooperatively engaging the surface 22 of a roadway uponwhich the marker is to be installed. Shell 12 is further provided with agenerally horizontal top 24, which is raised vertically above the base20, opposite front and rear faces 26 and 28, respectively, and oppositesides 29 which interconnect base 20 with top 24 to complete the shell12. The marker 10 is fixed in place upon the roadway surface 22 by anadhesive 30 which secures the marker at any desired location upon thepavement without requiring any recessing or other disruption of thepavement surface.

In order to render pavement marker 10 clearly visible during daylighthours, shell 12 is made opaque and is usually provided with a brightcolor. It is preferable, from the standpoint of durability, andespecially for maximum abrasion and wear resistance, to fabricate shell12 from a molded thermoplastic synthetic resin such as, for ex ample, anacrylic like methyl methacrylate or a polycarbonate, some of whichmaterials are known commercially as Lucite, Implex, or Lexan. Thesynthetic resin is pigmented to render shell 12 opaque and since thepigment runs completely through the resin, a certain amount of abrasionand wear will not affect the ability of the pavement marker to provide aclearly visible mark of desired color. Thus, front face 26 is located ina position to be viewed from oncoming vehicles and is provided with atleast a first portion 31, which is opaque and capable of beingeffectively viewed in daylight.

In order to render pavement marker 10- visible during night-time hours,a second portion 32. which is highly reflective is provided in the frontface 26 which faces oncoming vehicular traffic so that light emanatingfrom an oncoming vehicle will be reflected back toward the vehicle toestablish a marking clearly visible in the surrounding darkness.Reflecting portion 32 is constructed to be initially optically efficientand the trafiic marker is so designed as to maintain the opticaleffectiveness of the reflecting portion throughout the useful life ofthe marker. Thus, the body of the marker is provided with a windowfabricated of a synthetic resin of the same, or similar, nature as thematerial of shell 12 with the exception that the window islight-transmitting rather than opaque. The window is shown in the formof an insert 34 fitted into a corresponding gap 35 in the front face 26of the shell 12.

As best seen in FIGURES 3 and 4, insert 34 has an outer, generallyplanar, obverse light receiving and retracting surface 36. Aretro-directive reflector system 40 in the reverse surface 38 receiveslight emanating from an oncoming vehicle and incident upon the obversesurface 36 and reflects such light generally parallel to the directionof incidence so that the insert 34 is rendered refleca pavement markerconinven'tion is shown in and .has a body with tive and establishes thefacial reflecting portion 32 which renders the pavement marker capableof being efl'ectively viewed at night.

One of the most effective reflecting systems available employs thewell-known triple-mirror reflex reflecting principle which is thoroughlyexplained in the Stimson US. Patent No. 1,905,655, issued May 2, 1933,wherein there is disclosed a reflex light reflector including an obverselight receiving face and a reverse light reflecting face consisting of aplurality of cube corners, each having three surfaces adapted for totalinternal reflection of light impinging thereon from the obverse face.Each of Stimsons cube corners has an axis, and the cube corner axes areparallel to one another. For maximum optical efliciency, it is usual forsuch reflectors to be oriented with the cube corner axes essentiallyaligned with the direction of the nominal incident light. Since thelight emanating from oncoming vehicles is practically parallel to theroadway surface at distances where a pavement marker should provide amark visible from the vehicle, it would at first appear that use of theabove described reflecting system would necessitate having the obverseface of the reflector essentially perpendicular to the roadway surface.It has been observed, however, that such perpendicular surfaces willcollect dirt and a light impeding film will appear, which film will veryrapidly reduce the optical effectiveness of such a reflecting system.Thus, some means is required for periodically cleaning the obverse faceof the reflector.

It has been discovered that if the obverse face of the reflector systemis oriented horizontally, or parallel to the roadway surface, ratherthan vertically, the normal con tact between the tires of passingvehicles and the obverse face will wipe the face clean and the reflectorsystem can retain its optical effectiveness while in service.Unfortunately, however, such contact between oncoming vehicles and theobverse face will also give rise to abrasion of the surface of theobverse face which will rapidly deteriorate the optical quality of theface and reduce the optical effectiveness of the reflector. Suchabrasion can be reduced by orienting the obverse face perpendicular tothe roadway surface, or in a vertical plane. Thus, on one hand, theobverse face should be oriented toward the vertical to achieve initialoptical efficiency and to reduce optical deterioration arising out ofcontact with oncoming vehicles but should, on the other hand, beoriented toward the horizontal to allow the face to be periodicallywiped clean by such contact. Additionally, a planar face projectingvertically from the roadway surface could become a traffic hazard whilea horizontal face would reduce such a hazard.

Pavement marker 10 provides an optically effective reflecting systemwhich remains effective during service and still takes advantage of thewiping action arising out of contact with oncoming traflic. To this end,the generally planar obverse surface 36 of insert 34 is neither verticalnor horizontal, but is at an acute angle A to the roadway surface 22.Acute angle A is chosen so as to be great enough to place the reflectingportion 32 near enough to the vertical to allow the reflecting system tobe optically effective, but it is also small enough to allow adequatewiping of the obverse surface 36 by contact with the tires of oncomingvehicles. Acute angle A is also great enough to reduce opticaldeterioration of the obverse surface arising out of abrasion of thesurface by such contact with oncoming vehicles. Additionally, angle A issmall enough to prevent pavement marker 10 from becoming a traflichazard.

Good results in maintaining optical elfectiveness and in achievingadequate wiping action have been attained with an angle A of 30. Theoryindicates and experience has shown, however, that satisfactory resultscan be realized where angle A is varied within plus or minus of thenominal angle of Opaque portion 31 of front face 26 is also oriented soas to rise above the roadway surface 22 and makes an acute angle B withthe horizontal roadway surface. Angle B must also be chosen so as to begreat enough to place the opaque portion 31 near enough to the verticalto provide a sufliciently large area viewable from an oncoming vehicleto render the mark provided by marker 10 adequately visible in daylight.Thus, angle B must be great enough to establish a generally verticalprojected area viewable along the horizontal direction of sufficientlylarge size to provide the requisite viewable surface area. Additionally,angle B must be great enough to reduce deterioration of the opaqueportion 31 by wearing away of the surface of shell 12 through abrasivecontact with the tires of oncoming vehicles. At the same time, however,angle B must be small enough to enable adequate wiping of opaque portion31 by such contact with oncoming vehicles to maintain the surfaceportion clean. Additionally, angle B is small enough to prevent pavementmarker 10 from becoming a traflic hazard.

An effective magnitude for angle B has been found to be 20. Experiencehas shown, however, that satisfactory results can be realized whereangle B is varied within plus or minus 10 of the nominal angle of 20.

As best seen in FIGURES 1 and 2, opaque portion 31 may be comprised ofsmaller areas lying at various acute angles to the horizontal in orderto provide a relatively smooth transition between the insert 34, whichlies at the nominal angle A with respect to horizontal, and the majorarea 42 of the opaque portion 31, which major area lies at the nominalangle B which is ordinarily less than angle A. Thus, minor areas 44 and46 of opaque portion 31 will lie at acute angles having a magnitudebetween angles A and B.

Referring now to FIGURES 3 and 4, the reflecting system 40 is atriple-mirror reflex reflector system in principle, the reverse surface38 containing a plurality of light reflecting elements 50 each of whichis adapted for total internal reflection of light impinging on thereverse surface 38.

Each reflector element 50 has three substantially square planar surfaces52, 54 and 56 arranged mutually at right angles and meeting at a commonpoint 58 thus forming a cube corner 60. Each cube corner 60 has an axis(the diagonal of the cube passing through the cube corner), one of whichis illustrated at C in FIGURE 4. The axes of all cube corners 60 areparallel to one another, although they need not necessarily be parallel.

Light emanating from the headlamps of an oncoming vehicle on the roadwayis represented by ray L in FIG- URE 4 and is essentially parallel to thehorizontal surface of the road. In actuality, ray L will vary from thehorizontal by about only 6 while the vehicle passes through the zonewhere, the reflecting system is required to provide a mark visible fromthe vehicle. Because obverse surface 36 makes an acute angle A with thehorizontal roadway surface, ray L will be incident upon obverse surface36 at an angle of incidence i with the normal N to the obverse surfaceand will be refracted upon passage into the material of insert 32 at anangle of refraction 1' as the light proceeds toward reverse surface 38.Where the index of refraction of the material of insert 32 is u, lightray L will be refracted in accordance with the following formula:

sin i u= and the angle of refraction r can be expressed as:

r=sin- Z which may take place at the obverse face is generally of anegligible magnitude. Since maximum efliciency of such a reflector isrealized when the light impinging upon the reverse surface is parallelto the axes of the cube corners, maximum efficiency is easily attainedin ordinary reflector structures. However, in pavement marker 10,obverse surface 36 lies in a plane making a nominal angle of 30 with thehorizontal and angle of incidence i is approximately 60. The angle ofrefraction r then becomes sub stantial. For example, Where insert 32 ismolded of methyl methacrylate, u is 1.5 and the angle of refraction rwill be 3516. If the cube corner axes were aligned with the normal N tothe obverse face as in ordinary reflex reflector structures, the opticalefficienoy of the reflector system would suffer severely from the largeangle of incidence. However, by aligning the cube corner axes ofreflector elements 50 with the direction of the refracted light rays,that is, at an angle 1' to normal N, maximum efliciency of the reflectoris maintained despite the rather large angle of incidence.

It is noted that while ideally the cube corner axes should be alignedexactly parallel with the direction of the refracted light rays ascomputed above for maximum efliciency, in practice it has been foundthat adequate performance can be attained when the alignment of the cubecorner axes is within about 13 of the computed direction of therefracted rays. Hence, general alignment of the cube corner axes withthe refracted light within the above tolerances will give rise tosatisfactory operation of the pavement marker.

It will be apparent that the angle of incidence will increase with adecrease in angle A and the greater the angle of incidence, the greaterthe transmission losses at surface 36, both entering and leaving thesurface, and the greater the angular error of the return ray caused bysmall error in the reflecting cube. For optical reasons, then, as wellas for the reduction of abrasion arising out of contact with oncomingtrafiic, it has been determined that angle A should not be decreasedbelow 15.

The effectiveness of the reflector system 40 allows the reflectingportion 32 of front face 26 to occupy a relatively small portion of thearea of front face 26, leaving the larger area for opaque portion 31. Ingeneral, the opaque portion 31 should provide a significantly largerprojected viewable area than the corresponding area of reflectiveportion 32. It has been found that satisfactory results are attainedwhere the projected viewable area of opaque portion 31 is in the orderof magnitude of five times that of reflective portion 32.

It is noted that the trapezoidal configuration of insert 34 enablessides 62 and 64 (see FIGURE 2) of the insert to each follow a line ofcube corners 60 so that the entire insert is effective in providing areflective portion in the front face 26. Thus, sides 62 and 64 each makean angle of 75 with the base 66 of the trapezoidal configuration.

Shell 12 is readily fabricated by moldingan opaque portion of pigmentedthermoplastic synthetic resin utilizing well-known techniques andproviding the gap in the pigmented molded portion into which thelight-transmitting insert, which is also molded of a thermoplasticresin, is placed. Pavement marker 10 is then completed by filling themolded and assembled shell with a material which will adhere to innersurface 16 and harden to form solid core 14. Such materials as epoxycompounds have been found suitable for the formation of core 14,although other suitable cast synthetic resins will become apparent tothose skilled in the material arts. Since it is desirable from astructural standpoint that core 14 be contiguous with shell 12 and novoids exist between the shell and the core, reverse surface 38 is coatedwith a light-reflecting material 70, such as by metallizing the reversesurface in a now well-known manner, to assure that the reflectingelements 50 will perform their assigned function. Since the insert 34 ismolded separately from the remainder of the shell, the color andproperties of the insert are independent of the color and properties ofthe remainder of the shell and each may be fabricated with any desiredcolor or properties.

It will be noted that the overall appearance of the marker is pleasingas well as functional. The device presents no hazard to passing traflicsince both sides 29 as well as both faces 26 and 28 are at an angle tothe vertical which will reduce the shock imparted to the tires ofcontacting vehicles.

Although the pavement marker depicted in FIGURES 1 through 4 ismonodirectional in that only front face 26 is provided with a reflectiveportion 32, it will be apparent that a similar reflective portion couldbe provided in the rear face 28 to render the marker bidirectional.Thus, traffic marker 110, illustrated in FIGURE 5, is provided withfront and rear faces 126 and 128, respectively, each having an opaqueportion 131 and a reflective portion 132 to render traffic markerbidirectional.

It will be apparent that the illustrated! pavement markers are wellsuited to the attainment of the stated objects and advantages. Themarkers are self-cleaning without requiring moving component parts andwill exhibit exemplary performance over a relatively long useful life.

The above detailed description is provided by way of example only.Various details of design and construction may be modified withoutdeparting from the true spirit and scope of the invention as defined inthe appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A pavement marker for providing a marking on a generally horizontalroadway surface, the marking being visible from an oncoming vehicle onthe roadway both during the day and during the night, said pavementmarker comprising:

a body of synthetic resin having at least one portion providing a facelocated in position to be viewed from the oncoming vehicle, said bodyportion hava first part which is opaque, said first part providing afirst facial portion capable of being effectively viewed in daylight;"and a second part which is light transmitting and has an outer, obverselight receiving and refracting surface, an inner, reverse lightreceiving and reflecting surface, and a retro-directive reflectingsystem in said reverse surface for receiving light emanating from theoncoming vehicle and incident upon said obverse face and reflecting suchlight generally parallel to the direction of incidence for renderingsaid second part reflective and providing a second facial portioncapable of being effectively viewed at night;

each of said first and second facial portions being so oriented as tomake an acute angle with the horizontal and to rise above the roadwaysurface upon which the pavement marker is to be installed, each saidacute angle being great enough to reduce deterioration of said first andsecond facial portions arising out of contact with the oncoming vehiclewhile being small enough to allow adequate wiping of said first andsecond facial portions by such contact;

the acute angle of the first facial portion being great enough toprovide a sufliciently large projected viewable area; and

the acute angle of the second facial portion being great enough tomaintain adequate optical effectiveness of said retro-directivereflecting system during service.

2. The pavement marker of claim 1 wherein the acute angle of the firstfacial portion is within 10 of a nominal angle 20 and the acute angle ofthe second facial portion is within 15 of a nominal angle of 30.

3. The pavement marker of claim 1 wherein the acute angle of the majorpart of the first facial portion is 20 and the acute angle of the secondfacial portion is 30.

4. The pavement marker of claim 1 wherein the projected viewable area ofthe first facial portion is greater than the projected viewable area ofthe second facial portion.

5. The pavement marker of claim 1 wherein the projected viewable area ofthe first facial portion is in the order of magnitude of five times theprojected viewable area of the second facial portion.

6. The pavement marker of claim 1 wherein the body has a gap thereincontiguous with the first part thereof and the second part is in theform of an insert placed Within the gap.

7. The pavement marker of claim 1 wherein said retrodirective reflectingsystem is a triple-mirror reflex reflecting system in saidreversesurface.

8. The pavement marker of claim 1 wherein the body of synthetic resincomprises:

a shell of thermoplastic synthetic resin of given thickness;

a filler material of cast synthetic resin filling said shell andreinforcing the shell against forces applied externally by the contactwith the oncoming vehi cles;

said shell having at least a portion which is pigmented throughout thethickness thereof to render said portion opaque, said portion includingsaid first part of said body portion,

a gap in said shell in the facial portion of said body;

and

a light transmitting insert in said shell within the gap thereof andincluding the second part of said body portion.

9. The pavement marker of claim 8 wherein said retrodirective reflectingsystem is a triple-mirror reflex reflecting system including a pluralityof light reflecting elements in said reverse surface for effectingreflection of light impinging thereon from said obverse surface, eachsaid element having three planar surfaces arranged mutually at rightangles and meeting at a common point remote from said obverse surface toform a cube corner, the axis through the cube corner of each saidreflecting element being at an angle with the normal to said obversesurface for allowing said reflecting elements to receive light emanatingin a generally horizontal direction from the oncoming vehicle andretracted by said obverse surface as a result of the acute angle of thesecond facial portion and to reflect said light generally parallel tothe direction of incidence of the light incident upon said obversesurface.

10. The pavement marker of claim 9 wherein the acute angle of the firstfacial portion is within 10 of a nominal angle 20 and the acute angle ofthe second facial portion is within 15 of a nominal angle of 30.

11. The pavement marker of claim 9 wherein the acute angle of the majorpart of the first facial portion is 20 and the acute angle of the secondfacial portion is 30.

12. The pavement marker of claim 9 wherein the projected viewable areaof the first facial portion is greater than the projected viewable areaof the second facial portion.

13. The pavement marker of claim 9 wherein the projected viewable areaof the first facial portion is in the order of magnitude of five timesthe projected viewable area of the second facial portion.

References Cited UNITED STATES PATENTS 2,666,373 l/l954- Mattson 94-l.52,991,698 7/l961 Leubaz 94-1.5 3,093,038 6/1963 McRobbie 94-l.53,240,132 3/1966 Wiswell 94l.5 3,319,542 5/1967 Hansen 94 --1.53,332,327 7/1967 Heenan 94 1.5 3,343,467 9/1967 Bonvallet 941.5

NILE C. BYERS, JR., Primary Examiner.

Edward M. Fletcher, Jr.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,392,639 July 16, 1968 Sidney A. Heenan et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as show below:

Column 2, lines 67 and 68, "retracting" should read refracting Column 8,line 7, "retracted" should read refracted Signed and sealed this 30thday of December 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

1. A PAVEMENT MARKER FOR PROVIDING A MARKING ON A GENERALLY HORIZONTALROADWAY SURFACE, THE MARKING BEING VISIBLE FROM AN ONCOMING VEHICLE ONTHE ROADWAY BOTH DURING THE DAY AND DURING THE NIGHT, SAID PAVEMENTMARKER COMPRISING: A BODY OF SYNTHETIC RESIN HAVING AT LEAST ONE PORTIONPROVING A FACE LOCATED IN POSITION TO BE VIEWED FROM THE ONCOMINGVEHICLE, SAID BODY PORTION HAVING; A FIRST PART WHICH IS OPAQUE, SAIDFIRST PART PROVIDING A FIRST FACIAL PORTION CAPABLE OF BEING EFFECTIVELYVIEWED IN DAYLIGHT; AND A SECOND PART WHICH IS LIGHT TRANSMITTING ANDHAS AN OUTER, OBVERSE LIGHT RECEIVING AND REFRACTING SURFACE, AN INNER,REVERSE LIGHT RECEIVING AND REFLECTING SURFACE, AND A RETRO-DIRECTIVEREFLECTING SYSTEM IN SAID REVERSE SURFACE FOR RECEIVING LIGHT EMANATINGFROM THE ONCOMING VEHICLE AND INCIDENT UPON SAID OBVERSE FACE ANDREFLECTING SUCH LIGHT GENERALLY PARALLEL TO THE DIRECTION OF INCIDENCEFOR RENDERING SAID SEOND PART REFLECTIVE AND PROVIDING A SECOND FACIALPORTION CAPABLE OF BEING EFFECTIVELY VIEWED AT NIGHT, EACH OF SAID FIRSTAND SECOND FACIAL PORTIONS BEING TO ORIENTED AS TO MAKE AN ACUTE ANGLEWITH THE HORIZONTAL AND TO RISE ABOVE THE ROADWAY SURFACE UPON WHICH THEPAVEMENT MARKER IS TO BE INSTALLED, EACH SAID ACUTE ANGLE BEING GREATENOUGH TO REDUCE DETERIORATION OF SAID FIRST AND SECOND FACIAL PORTIONSARISING OUT OF CONTACT WITH THE ONCOMING VEHICLE WHILE BEING SMALLENOUGH TO ALLOW ADEQUATE WIPING OF SAID FIRST AND SECOND FACIAL PORTIONSSUCH CONTACT; THE ACUTE ANGLE OF THE FIRST FACIAL PORTION BEING GREATENOUGH TO PROVIDE A SUFFICIENTLY LARGE PROJECTED VIEWABLE AREA; AND THEACUTE ANGLE OF THE SECOND FACIAL PORTION BEING GREAT ENOUGH TO MAINTAINADEQUATE OPTICAL EFFECTIVENESS OF SAID RETRO-DIRECTIVE REFLECTING SYSTEMDURING SERVICE.